Multiple coil spring mems resonator

ABSTRACT

A multiple coil spring MEMS resonator includes a center anchor and a resonator body including two or more coil springs extending in a spiral pattern from the center anchor to an outer closed ring. Each pair of coil springs originates from opposing points on the center anchor and extends in the spiral pattern to opposing points on the outer ring. The number of coil springs, the length and the width of the coil springs and the weight of the outer ring are selected to realize a desired resonant frequency

CROSS REFERENCE TO OTHER APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/067,230 entitled MULTIPLE COIL SPRING RESONATORS, filed Oct. 22,2014, which is incorporated herein by reference for all purposes.

This application also claims priority to U.S. Provisional PatentApplication No. 62/067,206 entitled COMPOUND SPRING RESONATORS FORFREQUENCY AND TIMING GENERATION, filed Oct. 22, 2014, which isincorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

MEMS (microelectromechanical systems) resonators are smallelectromechanical structures that vibrate at precise frequencies. MEMSresonators are useful in electronic circuits for providing timingreferences and frequency references. In typical applications, a MEMSresonator is attached to an electronic circuit to form an oscillatorcircuit. A MEMS oscillator includes a MEMS resonator driven by asustaining amplifier in continuous motion. The mechanical resonantvibration of the MEMS resonator is sensed and converted into anelectrical signal with a very precise frequency. The precise MEMSresonant frequency is used as the reference frequency for the oscillatorcircuit. The electronic circuit attached to the MEMS resonator amplifiesthe sensed electrical signal and sets or adjusts the output frequency ofthe oscillator based on the MEMS resonant frequency. For example, theelectronic circuit may include a phase-locked loop (PLL) or afrequency-locked loop that generates programmable output frequenciesbased on the MEMS resonant frequency as the reference frequency.

Common applications for MEMS oscillators include real-time clocks. Areal-time clock (RTC) is a computer clock, often in the form of anintegrated circuit, used to keep track of the current time in electronicsystems, such as computers, servers and consumer electronic devices.FIG. 1, which includes FIGS. 1(a) and 1(b), illustrates conventionalreal-time clock circuits. Referring to FIG. 1(a), a real-time clock 1,often provided as a real-time clock integrated circuit, includes anoscillator circuit 2 and supporting circuitry (RTC circuit) 3.Traditional real-time clocks use a crystal oscillator circuit that usesthe mechanical resonance of a vibrating quartz crystal 4 to provide thedesired reference frequency. The quartz crystal 4, a discrete componentoutside of the real-time clock integrated circuit, is driven to resonateat a desired frequency, such as 32.768 kHz. The oscillator circuit 2turns the vibration of the quartz crystal 4 into an electrical signalwith the desired precise frequency (e.g. 32.768 kHz). The RTC circuit 3provides signal amplification, clock division, and other time keepingfunctions. The real-time clock 1 often includes an alternate powersource 5 so that the real-time clock can continue to keep time while theprimary source of power is off or becomes unavailable. The alternatepower source 5 can be a battery power source, such as a lithium ionbattery or a supercapacitor.

Because quartz crystal is bulky and does not integrate well withsemiconductor integrated circuits, MEMS resonators have become anattractive alternative to the traditional quartz crystal in constructingoscillator circuits. Referring to FIG. 1(b), a real-time clock 6 isformed using a real-time clock integrated circuit that includes areal-time clock chip 7 and a mems resonator 8, which can be co-packagedwithin the same integrated circuit package, such as, but not limited to,a quad flat no-leads package (QFN) or a land grid array package (LGA).The MEMS resonator 8 provides a precise reference frequency. Thereal-time clock chip 7 houses all of the supporting circuitry, includingthe oscillator circuit 2 and the RTC circuit 3. With the MEMS resonatorthus integrated, the size of the real-time clock is reduced.Furthermore, MEMS resonator provide additional benefits such as moreconsistent stability over a wider temperature range and betterresistance to environmental factors such as shock and vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1, which includes FIGS. 1(a) and 1(b), illustrates conventionalreal-time clock circuits.

FIG. 2, which includes FIGS. 2(a) and 2(b), illustrates a real-timeclock circuit and a MEMS oscillator circuit incorporating the MEMSresonator of the present invention in some embodiments.

FIG. 3, which includes FIGS. 3(a) and 3(b), includes a perspective viewand a top view of a compound spring MEMS resonator in embodiments of thepresent invention.

FIG. 4, which includes FIGS. 4(a) and 4(b), includes a perspective viewand a top view of a spring unit cell in embodiments of the presentinvention.

FIG. 5 is a perspective diagram of a compound spring MEMS resonatorincluding multiple resonator units in embodiments of the presentinvention.

FIG. 6 is a top view of the compound spring MEMS resonator of FIG. 5 insome embodiments.

FIG. 7 is a cross-sectional view of the compound spring MEMS resonatorof FIG. 6 along a line A-A′ in some embodiments.

FIG. 8, which includes FIGS. 8(a) to 8(c), illustrates the harmonicmotion of the MEMS resonator of FIG. 6 in some examples.

FIG. 9 is a perspective view of a compound spring MEMS resonatorincluding release holes in alternate embodiments of the presentinvention.

FIG. 10 is a perspective view of a multiple coil spring MEMS resonatorin embodiments of the present invention.

FIG. 11 is a perspective view of the multiple coil spring MEMS resonatorof FIG. 10 incorporating drive and sense electrodes in embodiments ofthe present invention.

FIG. 12 is a top view of the multiple coil spring MEMS resonator of FIG.11 in some embodiments.

FIG. 13 is a cross-sectional view of the multiple coil spring MEMSresonator of FIG. 12 along a line B-B′ in some embodiments.

FIG. 14, which includes FIGS. 14(a) to 14(b), illustrates the harmonicmotion of the MEMS resonator of FIG. 10 in some examples.

FIG. 15 is a perspective view of a multiple coil spring MEMS resonatorwithout release holes in alternate embodiments of the present invention.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; and/or a composition of matter. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

According to embodiments of the present invention, a compound springMEMS resonator includes a resonator body constructed using one or morespring unit cells forming a compound spring block and one or morecompound spring blocks forming the resonator body. Each compound springblock is anchored at nodal points to ensure a high quality (Q) factor.The resonator body further includes masses attached to the open ends ofthe compound spring block and capacitively coupled to drive/senseelectrodes. The dimensions of the spring unit cell, including the lengthand width of the beams forming the spring unit cell, the number ofspring unit cells for a compound spring block and the size and weight ofthe masses are selected to realize a desired resonant frequency.Meanwhile, the number of compound spring blocks and the aforementioneddimensional and configuration factors are selected to tune the desiredelectrical characteristics, such as impedance, of the MEMS resonator.

According to other embodiments of the present invention, a multiple coilspring MEMS resonator includes a center anchor and a resonator bodyincluding two or more coil springs extending in a spiral pattern fromthe center anchor to an outer closed ring. Each pair of coil springsoriginates from opposing points on the center anchor and extends in thespiral pattern to opposing points on the outer ring. The number of coilsprings, the length and the width of the coil springs, and the weight ofthe outer ring are selected to realize the desired resonant frequency.

In the present description, a MEMS resonator refers to a smallelectromechanical structure that vibrates at a stable and preciseresonant frequency. In embodiments of the present invention, the MEMSresonator is a silicon spring-mass system which can be excited intomechanical resonant vibration. A MEMS resonator is driven by asustaining amplifier to vibrate in continuous oscillation to generate anoutput frequency. In particular, the sustaining amplifier detects theresonator motion and drives additional energy into the resonator whilemaintaining the resonator motion at desired amplitudes. The resonantvibration is sensed and converted into an electrical signal having theresonant frequency of the resonator. The MEMS resonator has applicationsin forming MEMS oscillators and real-time clocks.

The MEMS resonators of the present invention realize many advantagesover conventional MEMS resonators. First, the MEMS resonators of thepresent invention are optimized for low frequency vibration.Conventional MEMS resonators configured for low frequency operationtypically require a large resonator body size. In embodiments of thepresent invention, the compound spring and coil spring MEMS resonatorsminimize the physical size of the resonator body while optimizing theresonator for low frequency output. Meanwhile, the MEMS resonators ofthe present invention are capable of realizing low motional impedancewhich is a critical parameter in oscillator or real-time clockapplication.

Second, the compound spring MEMS resonator formed using stacked springunit cells realizes low stiffness while maintaining good mechanicalstability in a compact area. The compound spring MEMS resonator of thepresent invention is stable in operation with predictable temperaturebehavior and achieves a high quality factor.

Lastly, the multiple coil spring resonator is anchored to the substratein the center. By using only a single center anchor, the resonancefrequency of the multiple coil spring resonator is less sensitive tosubstrate or package stress. The multiple coil spring resonator of thepresent invention therefore also achieves stability in operation with ahigh quality factor.

In some applications, the MEMS resonators of the present invention areused as a frequency source for an oscillator circuit. In particular, theMEMS resonators of the present invention can be used to provide a stableand accurate reference clock for real-time clock circuits to enable thereal-time clocks to maintain accurate time over temperature variations.In one example, the MEMS resonator of the present invention is used toconstruct a MEMS oscillator providing an output frequency of 32.768 kHzor a multiple of 32.768 kHz which is useful in real-time clock circuits.FIG. 2, which includes FIGS. 2(a) and 2(b), illustrates a real-timeclock circuit and a MEMS oscillator circuit incorporating the MEMSresonator of the present invention in some embodiments. Referring toFIG. 2(a), a real-time clock 10, provided as a real-time clockintegrated circuit, includes a real-time clock chip 11 and a MEMSresonator 12, either co-packaged in the same integrated circuit orformed on the same semiconductor substrate. The MEMS resonator 12 can beimplemented either as the compound spring MEMS resonator or the multiplecoil spring MEMS resonator described below. The MEMS resonator 12provides a precise reference frequency for the oscillator circuit 13formed on the real-time clock chip 11. For time-based applications, theMEMS resonator 12 is constructed to resonate at 32.768 kHz or at somemultiple of 32.768 kHz. For example, in some embodiments, the MEMSresonator 12 is constructed to resonate at 524.288 kHz (16 x) or at262.144 kHz (8 x). The phase-locked loop (PLL) of the real-time clockcircuit divides down the MEMS resonator frequency to the desired 32.768kHz frequency. In this manner, the MEMS resonator 12 can be made smallerto reduce cost while the real-time clock can improve noise during thefrequency division operation.

The real-time clock chip 11 houses all of the supporting circuitry forthe real-time clock 10 to provide signal amplification, clock division,and other time keeping functions. In the present example, the real-timeclock chip 11 includes the oscillator circuit 13 providing a stableclock signal, such as at 32.768 kHz. In particular, the oscillatorcircuit 13 drives the MEMS resonator 12 to generate a sensed signalhaving the desired reference frequency of 32.768 kHz or a multiple of32.768 kHz. The oscillator circuit 13 provides a clock signal having thefrequency of 32.768 kHz as the reference clock signal. The real-timeclock chip 11 further includes a clock divider 14 to generate an outputclock signal over a range of frequency based on the reference clocksignal. For example, the output clock signal can have a frequency of 1Hz to 32 kHz. The real-time clock chip 11 further includes control logiccircuitry 15 and a memory 16 to provide control and to realize othertime base or timekeeping functions. The schematic diagram of thereal-time clock 10 shown in FIG. 2(a) is illustrative only and is notintended to be limiting. In actual implementation, the real-time clock10 may include other circuitry not shown in the example schematicdiagram of FIG. 2(a).

The real-time clock 10 often includes an alternate power source 5 so thereal-time clock can continue to keep time while the primary source ofpower is off or becomes unavailable. The alternate power source 5 can bea battery power source, such as a lithium ion battery or asupercapacitor.

FIG. 2(b) illustrates an example construction of a MEMS oscillator insome embodiments. Referring to FIG. 2(b), the MEMS oscillator 13includes a sustaining amplifier 22 configured to drive the MEMSresonator 12 in continuous motion to generate an output frequency as thereference frequency. The reference frequency is the resonant frequencyof the MEMS resonator which is set at 32.768 kHz or a multipole of32.768 kHz. The MEMS oscillator 13 is further configured to sense theresonant vibration of the MEMS resonator 12. The sense signal isprovided to a phase-locked loop (PLL) 23 as the reference frequency. ThePLL generates a clock signal at the reference frequency or as a functionof the reference frequency. The oscillator circuit 13 may include anoutput driver 24 to generate the reference clock signal for use by thedownstream real-time clock circuitry. In the present example, thereference clock signal has a frequency of 32.768 kHz.

Compound Spring MEMS Resonator

FIG. 3, which includes FIGS. 3(a) and 3(b), includes a perspective viewand a top view of a compound spring MEMS resonator in embodiments of thepresent invention. Referring to FIG. 3, a compound spring MEMS resonator50 includes a resonator body 52 formed as a pair of connectedspring-mass systems 54 that is suspended above a substrate (not shown)and anchored to the substrate through a pair of isolating anchors 56. Inparticular, the resonator body 52 includes a first spring-mass system 54a (“spring-mass section 1”) and a second spring-mass system 54 b(“spring-mass section 2”) connected base-to-base and suspended above thesubstrate such that their relative motions and subsequent stressescancel at nodal points at which the spring-mass systems 54 are anchoredto the substrate through the isolating anchors 56.

In the spring-mass systems 54, each of the spring-mass sections 1, 2 isformed by a spring structure attached at the open end to a mass. Inembodiments of the present invention, each of the spring-mass sections1, 2 is formed using spring unit cells that are stacked to form acompound spring structure. In FIG. 3(b), the MEMS resonator 50 is shownwith the spring unit cell structure in a hatch pattern to illustrate thefolded spring structure of the spring unit cells. The compound springstructure may include one or more spring unit cells to obtain a desiredstiffness of the resonator and therefore to adjust the frequency of theresonator. More specifically, the number of spring unit cells can beselected to realize the desired resonance frequency for the MEMSresonator. The compound spring structures of the spring-mass sections 1,2 are connected at the base (the nodal point) to form a single compoundspring block 55. In spring-mass section 1, the compound spring structureis attached at the open end to a first mass 60 a. In spring-mass section2, the compound spring structure is attached at the open end to a secondmass 60 b. The size and weight of first and second mass 60 a,b areselected to tune or adjust the resonance frequency of the resonator.Furthermore, first and second mass 60 a,b function as the input andoutput electrodes of the MEMS resonator 50 to couple the input drivesignal and the output sense signal to/from the resonator body. The inputand output electrodes operate through capacitive coupling to enable thespring-mass systems 54 to be electrostatically driven and sensed, aswill be explained in more detail below.

In embodiments of the present invention, the spring-mass systems 54 areformed from spring unit cells 64 which are repeated and connected toform the compound spring structures. FIG. 4, which includes FIGS. 4(a)and 4(b), includes a perspective view and a top view of a spring unitcell in embodiments of the present invention. Referring to FIG. 4, aspring unit cell 64 has a compact folded spring structure. In thepresent example, the spring unit cell 64 has a rectangular folded springstructure. The spring unit cell 64 is designed to enable the foldedspring structure to be repeated by stacking to form a compound springstructure. The compound spring structure thus formed minimizes the sizeof the spring mass system while remaining stable. The compound springstructure formed from stacked spring unit cell is capable of providinglow stiffness while maintaining good mechanical stability in a compactarea in the MEMS resonator.

In embodiments of the present invention, the resonant frequency of theMEMS resonator 50 is defined by the dimensions of the spring unit cell,that is, the width (W) and the height (H) of the spring unit cell andthe length and width of the beams forming the spring unit cell. Theresonant frequency of the MEMS resonator 50 is further defined by thenumber of spring unit cells for a compound spring block and the size andweight of the masses.

Returning to FIGS. 3 and 4, in MEMS resonator 50, the resonator body 52includes the spring-mass section 1 and spring mass section 2 (54 a,b)that are connected base-to-base at the nodal points. The spring-masssections 1, 2 are anchored to the substrate at the nodal points byanchors 56 through isolating suspension beams 58. That is, the anchors56 are connected or attached to a substrate (not shown) and thespring-mass sections 1,2 are connected to the anchors 56 at the nodalpoints through the suspension beams 58. Suspension beams 58 are notattached to the substrate but are suspended above the substrate. In thepresent description, nodal points refer to points on the resonator bodythat do not move substantially or have only minimum movement duringresonant vibration. By anchoring the spring-mass systems 54 at nodalpoints, the MEMS resonator can achieve a high quality factor. Eachisolating suspension beam 58 connects the compound spring block 55 atthe nodal point to the anchor 56. The isolating suspension beam 58functions to isolate any residual motion or environmental stress fromthe anchor 56, thereby keeping the vibration energy in the spring-masssystem and improving the quality factor of the resonator. In alternateembodiments of the present invention, the suspension beams may beomitted and the anchors 56 may be directly attached to the resonatorbody at the nodal points.

In some embodiments, the MEMS resonator 50 is formed by patterning andetching a silicon layer having a thickness of 20-30 μm. Thus, the MEMSresonator 50 has a thickness of approximately 20-30 μm. The spring unitcell 64 has a width (W) of 56 μm and a height (H) of 15 μm and iscomposed of 3 μm wide beams forming the folded spring structure. In thepresent example, the spring-mass sections 1 and 2 (54 a and 54 b) areeach formed using three spring unit cells. The mass 60 a and 60 b eachhas a dimension of 56 μm by 22 μm. The suspension beam 58 has adimension of 13 μm by 30 μm. The anchor 56 has a dimension of 30 μm by30 μm. The resulting resonant frequency of the compound spring resonatoris approximately 524 kHz.

In embodiments of the present invention, the MEMS resonator 50 is formedusing a conductive material such as polysilicon or single crystallinesilicon. Furthermore, in some embodiments, the MEMS resonator of thepresent invention can be formed using standard CMOS fabricationprocesses. In some embodiments, the MEMS resonator is formed on asilicon-on-insulator (SOI) wafer. That is, the MEMS resonator is formedin a silicon layer formed on a substrate with an insulating layer formedthereon. For example, in one embodiment, a silicon base layer with 2 μmof silicon oxide formed thereon may be used as the substrate. Thesilicon layer, which can be a polysilicon layer or single crystallinesilicon layer, is formed on the substrate and the silicon layer ispatterned to form the MEMS resonator.

In some embodiments, the resonator body, including the spring-masssystems, the suspension beams and the anchors, can be formed by havingthe resonator structure lithographically patterned on the silicon layerformed on the substrate. In some embodiments, the silicon layer is asingle crystalline silicon layer and has a thickness of 20-30 μm. Thesilicon layer is patterned with the resonator structure, including thespring, the mass, the suspension beams and the anchors. Then, thesilicon layer is etched, such as using a wet etch process usinghydrofluoric acid, to release the resonator spring and mass structureexcept the anchors. After the etch process, the resonator body isreleased from the underlying substrate while the anchors remain attachedto the substrate. In some embodiments, release holes can be included inthe mass 60 a,b to facilitate the etching and release of the resonatorbody from the underlying substrate. The release holes also enable theweight of the mass to be adjusted to tune the resonant frequency of theMEMS resonator, as will be explained in more detail below.

In some embodiments, the MEMS resonator 50 is operated based onelectrostatic transduction. The MEMS resonator 50 forms narrow and wellcontrolled gaps with a drive electrode and a sense electrode where thedrive/sense electrodes are connected or attached to the substrate, asshown in FIG. 3(b). For example, a drive electrode 66 may becapacitively coupled to the mass 60 a while a sense electrode 68 may becapacitively coupled to the mass 60 b. The gap between the drive/senseelectrode and the mass is typically a small air gap, on the order of 1μm or less. Through the electrostatic transduced action, the MEMSresonator is driven into resonant vibration. The resonant vibration issensed and converted into an electrical signal having a well-defined andprecise frequency. Note that the resonator structure and the electrodesare symmetrical and therefore the drive and sense electrodes can beinterchanged.

In the embodiment shown in FIG. 3 described above, a MEMS resonatorincluding a single compound spring structure in the resonator body isdescribed. As described above, the dimensions of the spring unit cell,including the dimensions of the beams forming the spring unit cell andthe number of spring unit cells in the compound spring block areselected to tune the resonant frequency of the MEMS resonator.Furthermore, the size and the weight of the mass at the open ends of thecompound spring block are also selected to tune the resonant frequencyof the MEMS resonator. In some embodiments, the resonator body of FIG. 3represents a resonator unit where the resonator unit can be repeated andconnected in parallel to form a MEMS resonator having the desiredfrequency as well as electrical characteristics. In particular, whilefactors such as the spring unit cell dimensions, the number of springunit cells in a block and the size and weight of the masses contributeto the effective electrical characteristics of the MEMS resonator, theelectrical characteristics of the MEMS resonator can be further adjustedby connecting multiple resonator unit in parallel.

FIG. 5 is a perspective diagram of a compound spring MEMS resonatorincluding multiple resonator units in embodiments of the presentinvention. FIG. 6 is a top view of the compound spring MEMS resonator ofFIG. 5 in some embodiments. In FIG. 6, the MEMS resonator is shown withthe spring unit cell structure in a hatch pattern to illustrate thefolded spring structure of the spring unit cells used to form thecompound spring block. Referring to FIGS. 5 and 6, a compound springMEMS resonator 70 is formed using three resonator units 50 a, 50 b and50 c connected in parallel to form the resonator body. Each resonatorunit 50 a-c is constructed in the same manner as described above withreference to FIG. 3. More specifically, each resonator unit includes aspring-mass section 1 formed from three spring unit cells and aspring-mass section 2 formed from three spring unit cells. The compoundspring block for each resonator unit is anchored at the nodal points bythe suspension beams and the anchors. The anchors for two adjacentresonator units can be merged into a single anchor.

The mass 74 a and 74 b at the open ends of the compound spring blocksare formed as a single continuous or contiguous structure to realize theparallel connection. The mass 74 a is connected to the open end ofspring-mass sections 1 of all the resonator units. The mass 74 b isconnected to the open end of spring-mass sections 2 of all the resonatorunits. The mass 74 a is separated from the drive electrode 66 by anarrow air gap to form a capacitor with the drive electrode 66. The mass74 b is separated from the sense electrode 68 by a narrow air gap toform a capacitor with the sense electrode 68. The air gap between thedrive/sense electrode and the mass is small, typically on the order of 1μm or less. Note that the resonator structure and the electrodes aresymmetrical and therefore the drive and sense electrodes can beinterchanged. Furthermore, the drive electrode 66 and the senseelectrode 68 are connected or attached to the substrate while theresonator body, including the compound spring structure and the mass, issuspended above the substrate.

As thus configured, the compound spring MEMS resonator 70 realizesincreased electrode area with lowered effective impedance. Inparticular, the first and second masses 74 a,b form a large electrodearea for capacitive coupling to the drive electrode 66 and the senseelectrode 68, respectively. In particular, the mass 74 a at the firstopen end of the multiple parallely connected resonator units iscapacitively coupled to the drive electrode 66 to receive the inputdrive signal while the mass 74 b at the second open end of the multipleparallely connected resonator units is capacitively coupled to the senseelectrode 68 to provide the output sense signal. That is, the driveelectrode 66 is separated from the mass 74 a by a narrow and wellcontrolled gap so that the drive electrode 66 and the mass 74 a form acapacitor. Similarly, the sense electrode 68 is separated from the mass74 b by a narrow and well controlled gap so that the sense electrode 66and the mass 74 b form a capacitor. A large electrode area is madepossible by the parallel configuration of multiple resonator units.Furthermore, by having the resonator units thus connected in parallel,the resistance or motional impedance of the MEMS resonator is reduced,thereby improving the electrical characteristics of the MEMS resonator70.

Accordingly, in some embodiments, the compound spring MEMS resonator ofthe present invention can be formed by selecting the desired number ofspring unit cell for the compound spring block to tune the desiredresonant frequency of the MEMS resonator and by selecting the desirednumber of parallel resonator units to adjust the desired electricalcharacteristics for the MEMS resonator. In particular, the resonantfrequency of the MEMS resonator is tuned by the size (or dimension) andweight of the mass (mass 60 a,b) and the stiffness of the spring whichis determined by the number of spring unit cells and the dimensions ofthe spring unit cell, including the dimensions of the beams forming thespring unit cell. The impedance of the MEMS resonator is tuned by thenumber of parallely connected resonator units.

FIG. 7 is a cross-sectional view of the compound spring MEMS resonatorof FIG. 6 along a line A-A′ in some embodiments. Referring to FIG. 7,the MEMS resonator 70 is formed on a substrate including a silicon baselayer 76 and an insulator layer 77, such as silicon oxide, formed on thebase layer 76. The resonator structure is formed in a silicon layer 78,such as a single crystalline or polycrystalline silicon layer. Thesilicon layer 78 may be 20-30 μm thick. The silicon layer 78 ispatterned lithographically and then etched, such as a wet etch usinghydrofluoric acid, to release the spring structure. The anchors 56,being a large silicon structure, remain attached to the insulating layer77 as the undercutting from the wet etch is not sufficient to etchthrough the anchors.

FIG. 8, which includes FIGS. 8(a) to 8(c), illustrates the harmonicmotion of the MEMS resonator of FIG. 6 in some examples. Thedisplacement shown in FIG. 8 is exaggerated for illustrative purpose. Inactual practice, the movement of the MEMS resonator is small and remainswithin the narrow air gap between the mass and the drive/senseelectrodes. For example, when the air gap is 1 μm, the spring structureof the MEMS resonator has a displacement of less than 1 μm. Theoperation of the MEMS resonator 70 will be described with reference toFIGS. 6 and 8.

In embodiments of the present invention, the MEMS resonator 70 iselectrostatically driven and sensed. To actuate the MEMS resonator, a DCvoltage and a small AC signal is applied to the drive electrode 66. Themass 74 a is then driven capacitively through the air gap by the driveelectrode 66. As a result of the drive voltage, the compound springblock vibrates. More specifically, the compound spring structure in theMEMS resonator starts from a first position (FIG. 8(a)) and expandsoutward (or stretches) to push the mass at the two ends towards thedrive/sense electrodes (not shown), as shown in FIG. 8(b). With thecompound spring structure stretched to the maximum amplitude, thecompound spring structure returns to the first position, as shown inFIG. 8(c). The harmonic motion of the compound spring structure repeatsat the resonant frequency as the MEMS resonator 70 is being driven. Atthe nodal points where the anchor is attached to the compound springstructures, the compound spring structure does not move during thevibrational movement.

The displacement of the mass 74 b relative to the sense electrode 68alters the capacitance of the capacitor formed between the twoelectrodes. A time varying capacitor is formed between the mass 74 b andthe sense electrode 68. To sense the capacitance change, a DC voltage isapplied between the resonator structure and the sense electrode 68 andan AC current, indicative of the capacitance changes, is generated. TheAC current at the sense electrode is sensed to generate the sense signalhaving a precise frequency. In some embodiments, the MEMS resonator 70is tuned to a resonant frequency of 32.768 kHz or some multiple of32.768 kHz. The MEMS resonator 70 can be used to construct a MEMSoscillator providing an output frequency of 32.768 kHz which is usefulin real-time clock circuits.

FIG. 9 is a perspective view of a compound spring MEMS resonatorincluding release holes in alternate embodiments of the presentinvention. Referring to FIG. 9, a MEMS resonator 90 is constructed inthe same manner as MEMS resonator 70 of FIG. 5 but with the addition ofrelease holes 96 in the mass 94 a and 94 b at the open ends of thecompound spring block 95. The release holes 96 are used to adjust theweight of the mass 94 a, 94 b while also facilitate ease of fabrication.In particular, the weight of mass 94 a and 94 b can be tuned byincluding an arrangement of release holes 96 in the structure of themass. Adjusting the weight of the mass 94 a and 94 b adjusts the weightof the resonator body which in turn tunes the resonant frequency of theMEMS resonator. Furthermore, the release holes 96 enhance the wet etchprocess to release the resonator body by providing additional openingsfor the etchant to enter to etch under the resonant structure, therebyenhancing the etching process and ensuring the release of the resonatorstructure. The number, the size and the position of the release holes 96can be selected to both adjust the weight of the mass to tune theresonant frequency and to enhance the etching of the structure duringfabrication. The arrangement of the release holes in FIG. 9 isillustrative only.

The compound spring MEMS resonator of the present invention realizesmany advantages over conventional MEMS resonators. First, the compoundspring MEMS resonator of the present invention can realize a compactsize for low frequency as compared to conventional MEMS resonators.Second, the compound spring MEMS resonator of the present invention canrealize relatively low motional impedance which is a key parameter whenthe resonator is applied to construct timing or clock circuits, such asreal-time clocks. Third, the compound spring MEMS resonator of thepresent invention has symmetric resonant mode shape and has anchorsattached to nodal points of the resonant body. Accordingly, the compoundspring MEMS resonator of the present invention can achieve low loss andhigh quality factor.

Multiple Coil Spring MEMS Resonator

FIG. 10 is a perspective view of a multiple coil spring MEMS resonatorin embodiments of the present invention. Referring to FIG. 10, amultiple coil spring MEMS resonator 100 includes a resonator body formedby one or more pairs of coil springs 102 and an outer closed ring 104suspended above a substrate (not shown) and anchored to the substratethrough a center anchor 106. The coil springs 102 extend in a spiralpattern from the center anchor 106 to the outer closed ring 104. Eachpair of coil springs 102 originate from opposing points on the centeranchor 106 and extend in a spiral pattern to opposing points on theouter ring structure 104. In FIG. 10 and the following figures, the coilsprings of the MEMS resonator are shown in a different hatch pattern toillustrate the coil spring structure.

More specifically, in the present embodiment, a first coil spring 102 aoriginates from a first position on the center anchor and extends in aspiral pattern for a full circle around the center anchor 106 toterminate on the outer ring 104 at a position aligned with the firstposition. The second coil spring 102 b originates from a second positionon the center anchor that is opposite the first position. The secondcoil spring 102 b extends in a spiral pattern for a full circle aroundthe center anchor 106 to terminate on the outer ring 104 at a positionaligned with the second position. In embodiments of the presentinvention, the coil springs 102 can spiral in a clockwise direction orcounter-clockwise direction from the center anchor to the output closedring. A feature of the MEMS resonator of the present invention is theuse of long coil springs in the resonator body where the long coilsprings provide low spring constant in a very small area. In the presentembodiment, the coil springs extend in a spiral pattern in a full circlearound the center anchor. In other embodiments, the coil spring mayspiral in a partial circle around the center anchor. Furthermore, inother embodiments, the coil spring may spiral around the center anchorfor greater than a full circle.

The resonator body of MEMS resonator 100 further includes a set ofelectrodes 108 attached to the outer ring structure 104 and extendingoutward from the outer ring 104. In some embodiments, the electrodes 108are formed perpendicular to the outer ring. The electrodes 108 functionsas transducers to couple the drive and sense signals to and from theresonator body. In the present embodiment, eight transducers 108 areshown. In other embodiments, any number of one or more transducers 108may be incorporated. The use of eight transducers in the presentembodiment is illustrative only and not intended to be limiting.

In MEMS resonator 100, the number of the pairs of coil springs 102, thelength and width of the coil springs 102, and the weight of the outerclosed ring 104 are selected to realize the desired resonant frequency.In the embodiment shown in FIG. 10, the outer closed ring 104 and theelectrodes 108 are formed with release holes 130. Release holes 130 canbe used to adjust the weight of the outer ring and the electrodes so asto tune the resonant frequency of the MEMS resonator. The releases holes130 also function to facilitate the etching and release of the resonatorbody from the underlying substrate during the manufacturing process.

In alternate embodiments, the outer ring 104 may incorporate structuresother than release holes to adjust the mass and therefore adjust theresonance frequency. The structures on the external ring can be trimmedelectrically or by laser to adjust the resonator frequency withoutaffecting the quality factor of the MEMS resonator

In one embodiment, the weight of the outer ring 104 and the number ofpairs and the length and width of the coil springs 102 are selected sothat the MEMS resonator 100 functions as a low frequency resonator. Forexample, in one embodiment, the MEMS resonator 100 is configured for aresonant frequency of 32.768 kHz or some multiple of 32.768 kHz,suitable for timing application or real-time clocks.

In some embodiments, the MEMS resonator 100 is formed by patterning andetching a silicon layer having a thickness of 20-30 μm. Thus, the MEMSresonator 100 has a thickness of approximately 20-30 μm. The centeranchor has a diameter of 92 μm. The outer ring has an inner diameter of187 μm and a width of 12 μm. The coil spring has a width of 8.5 μm and alength that is greater than the circumference of the center anchor andcan be less than or greater than the inner circumference of the outerring. The resulting resonant frequency of the coil spring resonator isapproximately 64 kHz.

In embodiments of the present invention, the MEMS resonator 100 isformed using a conductive material such as polysilicon or singlecrystalline silicon. Furthermore, in some embodiments, the MEMSresonator of the present invention can be formed using standard CMOSfabrication processes. In some embodiments, the MEMS resonator is formedon a silicon-on-insulator (SOI) wafer. That is, the MEMS resonator isformed in a silicon layer formed on a substrate with an insulating layerformed thereon. For example, in one embodiment, a silicon base layerwith 2 μm of silicon oxide formed thereon may be used as the substrate.The silicon layer, which can be a polysilicon layer or singlecrystalline silicon layer, is formed on the substrate and the siliconlayer is patterned to form the MEMS resonator.

In some embodiments, the resonator body can be formed by having theresonator structure lithographically patterned on the silicon layerformed on the substrate. In some embodiments, the silicon layer is asingle crystalline silicon layer and has a thickness of 20-30 μm. Thesilicon layer is patterned with the resonator structure, including thecoil spring, the outer ring, and the center anchor. Then, the siliconlayer is etched, such as using a wet etch process using hydrofluoricacid, to release the resonator coil spring and closed ring structureexcept the anchors. After the etch process, the resonator body isreleased from the underlying substrate while the center anchor remainattached to the substrate. As shown in FIG. 10, release holes 130 can beincluded in the outer ring 104 to facilitate the etching and release ofthe resonator body. The release holes also enable the weight of the massto be adjusted to tune the resonant frequency, as explained above.

In embodiments of the present invention, the MEMS resonator 100 isoperated based on electrostatic transduction. The electrodes 108extending from the outer ring 104 form transducers for coupling to thedrive and sense electrodes. In some embodiments, the transducers can beconfigured for electrostatic comb drive. In other embodiments, thetransducers can be configured for electrostatic parallel plate drive.FIG. 11 is a perspective view of the multiple coil spring MEMS resonatorof FIG. 10 incorporating drive and sense electrodes in embodiments ofthe present invention. FIG. 12 is a top view of the multiple coil springMEMS resonator of FIG. 11 in some embodiments. Referring to FIGS. 11 and12, in the present embodiment, the transducers 108 of the MEMS resonator100 are configured for parallel plate drive. Each transducer 108 iscoupled to a set of drive electrode 110 and sense electrode 112. In thepresent example, the MEMS resonator 100 includes eight transducers 108.Therefore, a set of 16 drive/sense electrodes 110, 112 is provided forexcitation of the MEMS resonator.

Each pair of drive/sense electrodes are capactively coupled to arespective transducer 108. Each transducer is separated by a narrow andwell controlled gap with the drive electrode and the sense electrodewhere the drive/sense electrodes are connected or attached to thesubstrate. The gap between the drive/sense electrodes and the transduceris typically a small air gap, on the order of 1 μm or less. Through theelectrostatic transduced action, the MEMS resonator is driven intoresonant vibration. In particular, the coil springs and the outer ringrotate in clockwise and counter-clockwise directions. The displacementof the resonator body is less than the air gap, such as less than halfof 1 μm. The resonant vibration is sensed and converted into anelectrical signal having a well-defined and precise frequency. Note thatthe resonator structure and the electrodes are symmetrical and thereforethe drive and sense electrodes can be interchanged.

In embodiments of the present invention, the drive electrodes 110 can bedriven with a DC voltage and an AC signal having the same phase. Inother embodiments, the drive electrodes 110 can be driven with a DCvoltage and an AC signal in different phases.

FIG. 13 is a cross-sectional view of the multiple coil spring MEMSresonator of FIG. 12 along a line B-B′ in some embodiments. Referring toFIG. 13, the MEMS resonator 100 is formed on a substrate including asilicon base layer 120 and an insulator layer 122, such as siliconoxide, formed on the base layer 120. The resonator structure is formedin a silicon layer 125, such as a single crystalline or polycrystallinesilicon layer. The silicon layer 125 may be 20-30 μm thick. The siliconlayer 125 is patterned lithographically and then etched, such as a wetetch using hydrofluoric acid, to release the coil spring structure. Thecenter anchor 106, being a large silicon structure, remains attached tothe insulating layer 122 as the undercutting from the wet etch is notsufficient to etch through the anchors. The coil springs 102 and theouter ring 104 are suspended from the underlying substrate. The driveand sense electrodes 110 and 112 remain connected to the substrate.

FIG. 14, which includes FIGS. 14(a) to 14(b), illustrates the harmonicmotion of the MEMS resonator of FIG. 10 in some examples. Thedisplacement shown in FIG. 14 is exaggerated for illustrative purpose.In actual practice, the movement of the MEMS resonator is small andremains within the narrow air gap between the transducer and thedrive/sense electrodes. For example, when the air gap is 1 μm, the coilspring and outer ring structure of the MEMS resonator has a displacementof less than 1 μm and is typically on the order of half a micron. Theoperation of the MEMS resonator 100 will be described with reference toFIGS. 12 and 14.

In embodiments of the present invention, the MEMS resonator 100 iselectrostatically driven and sensed. To actuate the MEMS resonator, a DCvoltage and a small AC signal is applied to the drive electrode 110. Thetransducer 108 is then driven capacitively through the air gap by thedrive electrode 110. As a result of the drive voltage, the coil springand the outer ring rotate in a clockwise direction. More specifically,the coil spring and outer ring structure in the MEMS resonator startsfrom a first position (FIG. 14(a)) and rotates in a clockwise directionto a second position, as shown in FIG. 14(b). With the coil spring andouter ring structure thus rotated, the coil spring and outer ringstructure rotates in a counter-clockwise direction back to the firstposition, as shown in FIG. 14(a). The harmonic motion of the coil springand outer ring structure repeats at the resonant frequency as the MEMSresonator 100 is being driven. At the points where the coil springs areattached to the center anchor, the coil spring structure does not moveduring the vibrational movement.

The displacement of the transducer 108 relative to the sense electrode112 alters the capacitance of the capacitor formed between the twoelectrodes. A time varying capacitor is formed between the transducer108 and the sense electrode 112. To sense the capacitance change, a DCvoltage is applied between the resonator structure and the senseelectrode 112 and an AC current, indicative of the capacitance changes,is generated. The AC current at the sense electrode is sensed togenerate the sense signal having a precise frequency. In someembodiments, the MEMS resonator 100 is tuned to a resonant frequency of32.768 kHz or some multiple of 32.768 kHz. The MEMS resonator 100 can beused to construct a MEMS oscillator providing an output frequency of32.768 kHz which is useful in real-time clock circuits.

It is instructive to note that the MEMS resonator 100 has a symmetricalresonator structure and therefore the drive and sense electrodes can beinterchanged. Furthermore, the coil spring and outer ring structure canrotate from counter-clockwise direction to clockwise direction and thenrepeats. The order of the clockwise and counter-clockwise rotation isnot critical to the practice of the present invention.

FIG. 15 is a perspective view of a multiple coil spring MEMS resonatorwithout release holes in alternate embodiments of the present invention.Referring to FIG. 15, a MEMS resonator 150 is constructed in the samemanner as MEMS resonator 100 of FIG. 10 but without any release holes.The multiple coil spring MEMS resonator of the present invention can beformed using release holes as shown in FIG. 10 to adjust the weight ofthe outer ring and the transducers. However, the release holes areoptional. In the embodiment shown in FIG. 15, the multiple coil springMEMS resonator can be formed without release holes. In otherembodiments, the multiple coil spring MEMS resonator can be formed withrelease holes in the outer ring but not on the transducers. In yet otherembodiments, the multiple coil spring MEMS resonator can be formed withrelease holes in the transducers but not on the outer ring.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A MEMS resonator comprising: a center anchorconnected to a substrate; a resonator body comprising one or more pairsof coil spring and an outer closed ring, the coil spring extending in aspiral pattern from the center anchor to the outer closed ring, thecenter anchor is at a nodal point of the resonator body; one or moretransducers formed on the outer closed ring, each transducer beingformed perpendicular to and extending outward from the outer closedring; a set of drive electrodes and a set of sense electrodes formedattached to the substrate, each transducer being capacitively coupled toone drive electrode and one sense electrode.
 2. The MEMS resonator ofclaim 1, wherein a number of the pairs of coil springs, a length and awidth of the coil springs, and a weight of the outer closed ring areselected to tune a resonant frequency of the MEMS resonator
 3. The MEMSresonator of claim 2, further comprising: structures formed on the outerclosed ring to adjust the weight of the outer closed ring.
 4. The MEMSresonator of claim 1, wherein a respective drive electrode and arespective transducer is separated by a first gap and the senseelectrode and a respective sense electrode and a respective transduceris separated by a second gap, each of the first and second gaps beingequal to or less than 1 μm.
 5. The MEMS resonator of claim 1, whereinthe resonator body has a thickness of 20-30 μm.
 6. The MEMS resonator ofclaim 1, wherein the resonator body is formed from a material selectedfrom a single crystalline silicon layer and a polycrystalline siliconlayer.
 7. The MEMS resonator of claim 1, wherein the substrate comprisesa silicon base layer on which an insulating layer is formed.
 8. The MEMSresonator of claim 1, wherein each of the coil springs originates from afirst position on the center anchor and extends in a spiral pattern fora full circle around the center anchor to terminate on the outer closedring at a position aligned with the first position.
 9. The MEMSresonator of claim 1, wherein each pair of coil springs originate fromopposing points on the center anchor and extend in a spiral pattern toopposing points on the outer ring structure.
 10. The MEMS resonator ofclaim 1, wherein each of the coil springs extends in a clockwise spiralpattern from the center anchor.
 11. The MEMS resonator of claim 1,wherein each of the coil springs extends in a counter-clockwise spiralpattern from the center anchor.
 12. The MEMS resonator of claim 1,wherein the outer closed ring comprises release holes formed therein,the release holes adjusting the weight of the outer closed ring.
 13. TheMEMS resonator of claim 1, wherein the one or more transducers compriserelease holes formed therein, the release holes adjusting the weight ofthe one or more transducers.
 14. The MEMS resonator of claim 1, whereina number of spring unit cells in the compound spring structure and asize and a weight of the mass in each spring-mass section are selectedto tune a resonant frequency of the MEMS resonator.
 15. The MEMSresonator of claim 1, wherein the one or more transducers are driven byrespective drive electrodes by drive signals in different phases.